1. Field of the Invention
The present invention pertains to a kaolin clay-based pigment and more particularly to a kaolin clay pigment which contains an added inorganic aluminum oxide compound and which finds utility as a coating material or filler for paper of various kinds.
Still more particularly, the invention pertains to pigments of enhanced brightness made from kaolin clays containing iron compounds, for example, iron oxides and/or iron oxides in conjunction with titanium oxides. One such class of kaolin clays containing iron compounds is "hard kaolin clay". This term, as used in this specification and in the claims, means kaolin clays such as the sedimentary clays mined in the middle and east Georgia kaolin districts. These clays are distinguished from the more commonly known and used soft kaolin clays in a number of ways as summarized, for example, in table form at page 29 of "Field Conference, Kaolin, Bauxite, Fuller's Earth, Annual Meeting of the Clay Minerals Society, 1979", the disclosure of which is incorporated by reference herein. Soft kaolin clays usually have a white or sometimes orange tint and display the characteristics of fracturing smoothly and breaking easily into friable angular fragments when dried, the fragments displaying smooth, flat or sub-conchoidal surfaces. In contrast, most but not all hard kaolin clays have a gray tinge or coloration and are further characterized by breaking into irregularly shaped fragments having rough surfaces. The hard kaolin clays usually have a higher dry strength than do soft kaolin clays at the same moisture content, which accounts for the nomenclature "hard" kaolin. Further, hard kaolin clays as a class are finer than the soft kaolin clays. For example, whereas about 50% by weight of the particles of a typical hard kaolin crude clay are finer than 0.4 micron, about 50% by weight of the particles of a typical soft kaolin crude clay are finer than 1.5 microns, as determined by conventional sedimentation procedures. Hard kaolin clays also commonly contain a significant iron content, typically about 1% by weight Fe.sub.2 O.sub.3 whereas soft kaolin clays ordinarily contain an appreciably lesser quantity of iron compounds, for example, about 0.3% by weight Fe.sub.2 O.sub.3. Many differences between hard kaolins and soft kaolins are thought to be traceable to differences in their origin. Soft kaolins are believed to have been formed by in situ weathering of kaolinite crystals. Hard kaolin deposits, on the other hand, are believed to have been formed by the sedimentation from marine water of transported kaolinite crystals, this sedimentation taking place under anoxic conditions.
Hard and soft kaolin clays are distinguished from each other in the text Grim's "Applied Clay Mineralogy", 1962, McGraw-Hill Book Company, Inc., at pages 394 to 398 thereof, the disclosure of which is incorporated by reference herein. As mentioned in the Grim publication, hard kaolins are generally darker than soft kaolins. The Grim text also points out that the ultimate size of particles, i.e., the size of the particles in a well-dispersed clay pulp, of hard kaolin clays is significantly finer than those of soft kaolin clays. As described by the Grim text, a representative sample of hard kaolin clay had about 90% by weight of the ultimate size particles finer than 2 microns and about 60% by weight finer than 1/2 micron, the average particle size of typical hard clays being below 1/2 micron. Soft kaolin crude clays, in contrast, contain a substantial amount of particles coarser than 2 microns, with the average particle size of a representative papermaking soft kaolin clay being about 1 micron, with only a minor amount finer than 1/2 micron. Such particles generally differ from the finer particles in that the former are composed of a substantial proportion of stacks or booklets of hexagonal clay crystals. Still other stated differences in the Grim text between hard and soft clays are that hard kaolin clays tend to be less ordered (well crystallized) than soft kaolin clays and therefore produce less sharply defined X-ray diffraction peaks, and the hard kaolin clays absorb less water than do soft kaolin clays. Kaolinite is a major constituent of kaolin clays.
For many commercial uses, such as for use as a coating or filler on paper stock, the brightness of the pigment is significant. The brightness values of such materials are conventionally measured by a GE block brightness test and all brightness values of pigments and other materials referred to herein are as determined by TAPPI (The American Paper and Pulp Institute) procedure T64605-75. The material to be tested for brightness is pressed into a ring-shaped sample holder and its brightness is measured by using a GE reflectance meter and light having a wavelength of 457 nanometers.
The abrasiveness of the material used as a coating or filler on paper stock is also important as more abrasive materials increase the rate of wire wear on the papermaking machine. The weight loss of a wire screen caused by the abrasive action of a given coating or filler material may be employed as a test to indicate a relative measure of the abrasive characteristic of the material. One such well known test, referred to as the Valley abrasion test, is similar to Procedure 65, Institute of Paper Chemistry, April, 1962 (TAPPI 32 (II) 516-519) and was used to measure abrasiveness as referred to herein below.